Should I Ally the Grox?

if you want total galactic conquest, ally with the grox. If you want to play the part of the good guy, fight them. Remember, you can pay allies to attack the Grox and they won't destroy colonies. Grox are much more powerful than other empires.

What happens if you befriend the Grox?

Allying with the Grox will get you the "Dance With The Devil" badge. It really helps to be a Zealot, as you get a weapon that both takes over a planet *and* breaks the Galactic Code. ... When you'll ally the Grox, there will be a lot of war declarations around the galaxy. Just exit and save your game.

Can you eradicate the Grox?

So once you make it to the center and get the Staff of Life, then you can destroy the Grox. Simply go to one of their planets and use the staff of life on it, their only colony on it will be destroyed. Then place a Monolith on the planet, that will help the lifeforms on the planet.